Experimentelle Untersuchung strukturabhängiger Vorgänge am Beispiel der Oberflächen-Selbstdiffusion und der Adsorption auf Pt(100)

Freyer, N.

Book/Report

FZJ-2018-02536

Experimentelle Untersuchung strukturabhängiger Vorgänge am Beispiel der Oberflächen-Selbstdiffusion und der Adsorption auf Pt(100)

Freyer, N. (Corresponding author)FZJ*

1985
Kernforschungsanlage Jülich, Verlag Jülich

Jülich : Kernforschungsanlage Jülich, Verlag, Berichte der Kernforschungsanlage Jülich 2003, 133 p. (1985)

Please use a persistent id in citations: http://hdl.handle.net/2128/18227

Report No.: Juel-2003

Abstract: The correlation of two basic processes on single crystal surfaces with the structure of the surface was studied, namely surface self-diffusion and adsorption on Pt(110) . This surface normally exhibits a (1x2)-reconstruction instead of the (1x1)-structure expected from a simple termination of the bulk. The (1x2) surface is described by the "missing-row" model according to the most recent investigations. The diffusion measurements were done in comparison to the non-reconstructed Cu(110) surface in order to find out whether there is some characteristic difference between surfaces that exhibit a reconstruction and these that do not. Special interest was focused on the directional anisotropy of diffusion on this surface as measurements with field ion microscopy at low temperature had shown an unusual behaviour for reconstructing fcc(110) surfaces. The adsorption experiments consisted of site-specific XPS-measurements and a quantitative evaluation of the coverage in order tofind a correlation with the structure of the surface. For that purpose the adsorption properties of different modifications of the Pt(110) surface, namely the (1x2)- and the (1x1)-surface, were investigated. The temperature dependence of the diffusion constant for surface selfdiffusion was measured via the change of a sinusoidal profile in the surface in the temperature region of 0.6 - 0.9 $\Gamma_{m}$ ($\Gamma_{m}$ = melting point) for Pt(110) and Cu(110). In both cases a directional anisotropy was found whereas field ion microscopy had shown isotropic diffusion at a temperature of about 100 K for Pt(110) despite the twofold symmetry of the surface. The mass transport activation barriers for Cu(110) were found to be 1.1 eV for diffusion in [110]-direction and 2.6 eV for diffusion in [001]-direction. For Pt(110) the activation barriers were measured as 1.7 eV for the [110]-direction and 3.2 eV for the [001]-direction. The comparison of the measured data with theoretical values showed good agreement so that a more detailed explanation of the diffusion processes can be given. The diffusion is dominated by the movement of adatoms or dimers in the temperature region that was investigated. Especially no direct and significant influence of the reconstruction of the surface was found. However, an unusual behaviour of the decay of the surface profile was observed in the case of Pt(110) in the [001] diffusion direction. A comparison with the theory for this decay process showed that this behaviour is due to the orientation dependence of the surface tension. As this property in connected with the surface reconstruction it points to a correlation between reconstruction and decay rate of the profile. In addition the measurements with Cu give indication for a phase transition of the (110)-surface near 1050 K. In the adsorption experiments we used O$_{2}$, CO and NO as adsorbates and measured the O(ls)-signal with XPS. After O$_{2}$-adsorption molecular as well as atomic oxygen could be found where molecular oxygen exists only below room temperature. For adsorption on the (1x2)-surface the XPS-measurements together with 0-measurements yielded the distribution on different adsorption sites and the order of occupation. By comparison to the measurements on the (1x1)-surface it could be shown that the close-packed [110]-rows are primarily responsible for the dissociation of the oxygen molecules. A reactivity order of different Pt-surfaces concerning the oxygen dissociation is given. The other two adsorbates CO and NO have a similar influence on the structure of the Pt(110)-surface as both lift the (1x2)-reconstruction of the clean surface. Despite these similarities, however, both gases behave differently concerning the occupation of different adsorption sites, the saturation coverage and the work function change. For CO the adsorption process could be followed via site-specific O(1s) binding energy and measurements, quite analogous to O$_{2}$ adsorption. Bridge and on top bonded CO molecules could be distinguished. In particular for the c(8x4)-LEED pattern occurring at low temperatures a structure model has been proposed. The contributions of different adsorption sites, however, could not be separated in the case of NO. In the NO-adsorption no dissociation could be observed in contradiction to previous publications. Summarizing, the adsorption measurements show that it is possible to do structure sensitive investigations with XPS. Although the results would not allow a definite decision for one or another structure model of the Pt(110)(lx2)-surface, the measurements can be consistently explained by assuming the "missing-row" model and therefore support this model.

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